The present invention relates to a side printing head assembly which, in photographic film manufacturing processes, prints characters, symbols or the like on the marginal portion of film web, and more particularly to a side printing head assembly which applies a multicolor exposure light to the marginal portion of film web.
Side printing head assemblies are usually used in the manufacture of photographic film to apply latent images of information such as manufacturer's name, product code, frame number, and/or DX bar code along the longitudinal margin or margins of a film.
In a conventional side printing assembly, as is shown in FIG. 5, a strip of photographic color film 1 to which a latent image should be applied is transported at a constant speed by means of a roller 2. For an accurate transportation of the film strip 1, the roller 2 is control-led to rotate by a control circuit 4 which receives a signal representative of the number of revolutions of the rotary shaft 2a of the roller 2 through a pulse generator 3. In synchronism with its accurate advance, the photographic color film 1 is exposed.
The exposure is effected by means of a printing head which is comprised by an illumination source 5 having light emitting diodes (which are referred hereinafter to as LEDs) 6, 7 and 8 for red, green and blue, respectively, light guide means 9 comprising a large number of optical fibers for transmitting light from the illumination source 5 to an exposure station, and a focussing lens 11 for focussing the light from the light exit 10 of the light guide means 9 onto the photographic color film 1.
The light guide means 9 is, as is seen in FIG. 5, provided one for each LED 6, 7, 8 for red, green, blue. At the light exit 10, red, green, and blue lights are mixed for each picture element. Due to the provision of the light guide means 9 each having three light incident ends for three colors for each picture element to mix different colors of lights emitted from a plurality of LEDs, a desired color of character or symbol can be printed on the photographic color film in the form of a latent image.
By contrast, in a conventional side printing assembly, the provision of light guides each having only one light incident end for each color gives rise to a problem in that it becomes noticeably difficult to arrange the light guides snugly between the illumination source 5 and the light exit 10 and to form the light exit 10. As a result, the printing head cannot be made small and at a low cost.
Another problem which the conventional side printing assembly encounters is that it can produce either a focussed image of uneven color, or else an unclear image with even color; but it cannot produce a focussed image of even color. Specifically, because of the mixed printing light to which the photographic color film is exposed, unevenness in color occurs when an image is sharply focussed on the photographic color film and, on the other hand, an uunclear image is formed on the photographic color film if steps are taken to avoid unevenness in color.
Moreover, when using optical fiber bundles for the light guide 9, there is a problem of low tight transmittance between the LED and the optical fiber bundle as compared to a single optical fiber. Therefore, if the light guide 9 comprising the optical fiber bundles is snugly arranged between the illumination source 5 and the light exit surface 10, there arises a problem that many fibers are broken and that the light transmitting efficiency is lowered, making it difficult to provide an illumination light with a high luminance. In the conventional side printing assembly in which the light guide has three light incident ends, it is quite difficult to repair or replace a broken light guide.